This invention relates generally to electronic switches, and particularly to those well suited for on-off control of a variety of electric circuits notably including matrix converters and AC power supplies. More particularly, the invention deals with a solid-state, monolithic switching device that lends itself to use as a bidirectional, or alternating current (AC), switch.
Perhaps the most familiar example of AC switches is a bidirectional triode thyristor, better known as a TRIAC (TRIode for Alternating Current). The TRIAC has the weakness that, once triggered, it remains conductive until the current flowing therethrough drops below a predefined threshold value. It cannot possibly be turned off at a controllable point in an AC cycle.
Some bidirectional switching devices have been known which meet that requirement. One of them (shown in FIG. 1 of the drawings attached hereto) is a multifunction bidirectional switch capable of operation in four different modes to be set forth later in this specification. It comprises two insulated-gate bipolar transistors (IGBTs) connected in inverse series with each other, and two diodes connected in inverse parallel with the respective IGBTs. The two IGBTs are replaceable with insulated-gate field-effect transistors (IGFETs), junction gate field-effect transistors (JFETs), or bipolar transistors. A further known bidirectional switch is an inverse parallel connection of two IGBTs.
These prior art bidirectional switching circuits, all incorporating discrete normally-off electronic switches, are alike in being unnecessarily bulky and complex in construction and high in on-state voltage and resistance. Moreover, being made from semiconducting silicon materials with relatively low bandgaps, such known devices are not so high in antivoltage strength as can be desired.
Patent Cooperation Treaty International Publication Number WO 2005/079370 A2 by International Rectifier Corporation teaches a III-nitride bidirectional switch in the form a monolithically integrated circuit free from the noted drawbacks of the more conventional devices above. It has a first semiconductor body of GaN and, thereover, a second semiconductor body of AlGaN. A pair of ohmic power electrodes (interchangeable source and drain) overlie the second semiconductor body in ohmic contact therewith. A gate electrode is disposed between the ohmic electrodes and makes a schottky contact with the second semiconductor body. Being of symmetrical make, this known device is capable of on/off operation when either of the two ohmic electrodes is higher in potential than the other, that is, when either a positive or a negative voltage is applied therebetween.
The prior art III-nitride bidirectional switch above offers the benefit of less manufacturing costs through monolithic integration of a minimal number of constituent parts. Another strength is a higher antivoltage strength earned by use of semiconducting nitrides or other compounds for the semiconductor bodies. Offsetting these benefits, however, is the fact that the device is materially less versatile in use than the multifunction bidirectional switch first cited above, the former being incapable of operation in two of the four different modes possible with the latter. A more in-depth discussion will be given subsequently on this subject as it concerns the very gist of the instant invention.
There is another circumstance that has been taken heed of in developing the instant invention. That is a strong customer demand for stabilization of the potential of the substrate supporting the semiconductor bodies in the second cited bidirectional switch and other devices of comparable design. To that end, in the known high electron mobility transistor (HEMT) for example, the source is electrically connected to the substrate. This solution is impossible, or at least undesirable, in electronic switches of symmetrical design now under consideration, in which each of the two ohmic electrodes may serve as either source or drain.